The present invention relates to a method for injection moulding an external housing of an object, said object comprising an electronic assembly.
In particular, said method relates to an injection moulding method for a high temperature or harsh environment RFID tag or wireless device. Furthermore, the present invention relates to a molding apparatus and to an object obtained by said injection moulding method.
RFID tags and RFID systems are widely used and are well-known in the arts for several identifying, monitoring and controlling applications. Such RFID tags usually comprise an electronic assembly defined by a substrate carrying an IC circuit and an antenna. Said antenna transmits/receives from said IC circuit to an external device, host or computer system which interfaces to the RFID tag via an RF link.
Besides RFID tag, wireless electronic devices such as wireless sensors have been developed recently. A wireless sensor network of spatially distributed and cooperating autonomous wireless sensors is able to monitor physical or environmental conditions in many industrial and civilian application areas, including industrial process monitoring and environment control. Each wireless sensor includes electronics on a substrate, that is an electronic assembly, and an antenna for wireless communication and thus the structure of said sensors can be assimilated to the structure of RFID tags.
The electronic assembly is usually received inside an external housing so that the RFID tag or the wireless device is protected from being unnecessarily broken or damaged. Moreover said external housing allows the RFID tag or the wireless device to be encapsulated for durability against shock, fluid or dust and to be attached to a product.
As far as RFID tags are concerned, RFID tags are packaged on a variety of different forms to comply with different needs. The material with which said external housing is built, usually stiff plastics for harsh environment application, allows said tag to be adhered to a product by glue or instead to be fastened by a multitude of fastening elements. In case the RFID tag is packaged such as to be fastened to a product, a through hole is provided on said RFID to receive a shank of a fastening element, for example a shank of a screw. Said through hole of said RFID tag is obtained with a mounting through hole in said external housing and with a correspondent bigger further mounting through hole in said electronic assembly, which are aligned in a manufacturing step of said RFID tag. It is thus possible to obtain an RFID tag in which said external housing encapsulates said further through hole and provides a region around said through hole in which said electronic assembly is not present, such that only said external housing is stressed when said tag is fastened to a product.
For both RFID tags or wireless electronic devices, it is known manufacturing their external housing by mounting a top of plastics over a base of the same material, thus forming a chamber therein. Said electronic assembly is placed during manufacturing over said base and enclosed inside said chamber when said top is sealed on said base in a two stage process. Usually, said top is over moulded to said base or adhesive sealed, defining a welding border.
In case a through hole is needed in the object, said through hole in the top, in the bottom and in said assembly is machined in a first process stage and then subsequently said top, bottom, and assembly are combined in said two stage process. Therefore, said through hole defines a further welding border in said objects.
The external housing of said objects is critical in certain applications. For example, although protecting from most environmental factors, high temperature and/or harsh chemical environments may adversely affect plastics with which said external housing is built. Moreover, welding borders represent discontinuity points for said external housing and are often subjected to cracks, from which liquids, water or chemicals, may penetrate into the inside of the object. Furthermore, in case of objects with a through hole fastened in operation to a product by a screw, said further welding border is even more stressed, due to the pressing of said screw on plastics around said through hole, such that the discontinuity represented by said further welding border is critical.
From U.S. Pat. No. 5,973,599, is known an improved high temperature tag RFID, in which an electronic assembly is placed inside an external housing comprising a thermally resistant plastics, namely plastics with glass fibres, also called glass filled plastics. The housing top is not sealed over the base since it is sealably injection moulded to the housing base, encapsulating the assembly therein. The RFID tag of U.S. Pat. No. 5,973,599 also comprises a central through hole that allows the tag to be either fastened or glued on the product.
The addition of glass to plastics yields a very strong, yet lightweight material, that improves characteristics of plastics, such as resistance to high temperature and chemicals, but makes plastics more difficult to be machined. Due to the fact that glass fibres in plastics have a melting point much higher than the melting point of plastics, closer to the melting point of glass, the higher is the percentage of glass in plastics, the higher is the difficulty, at the usual injection moulding temperature of plastics, of a layer of glass filled plastics to over mould another layer of glass filled plastics in a two stage injection moulding process. As a result of the over moulding, portions of the mating surfaces between the two over moulded layers do not melt together, causing discontinuity points in the plastics of the external housing of the object which evolve during operation, giving rise to micro cracks inside said plastics that lead to leak paths from which liquids, water or chemicals, may penetrate into the inside of the object.
An object of the invention is to solve the aforementioned drawbacks, providing an object comprising an electronic assembly enclosed in an external housing made of plastics, in which said external housing is improved.
In particular, a further object of the invention is to provide an object comprising a through hole in which plastics of said external housing around said through hole are further improved.
A further object is to obtain an improved injection moulding method that eliminates defects in the over moulding or adhesive sealing of the external housing of said object, such that said external housing provides an improved resistance to high temperature and chemicals.
A further object is to provide an injection moulding apparatus allowing said method to be achieved.
In a first aspect of the invention, a method of injection moulding a plastics external housing of an object according to claim 1 is provided.
In a second aspect of the invention, an object according to claim 22 is provided.
In a third aspect of the invention, an apparatus according to claim 27 is provided.
Owing to these aspects of the invention it is possible to manufacture an object comprising an electronic assembly encapsulated inside an external housing made of plastics in which said external housing is a single continuous body of plastics. In other words, said plastics are integrally moulded and devoid of discontinuity points without neither welding borders nor mating surfaces.
In particular, by interposing said assembly between a first half-mould and a second half-mould defining a moulding cavity, and injecting plastics having positioned movable support means holding said assembly inside said injection cavity, it is possible to inject plastics all around said assembly at the same time. Furthermore, said support means is disengaged from said assembly before said plastics are cured, thus manufacturing said external housing is obtained with only one shot process step that encapsulates said assembly on an external body of plastics in which plastics are a single continuous body. In other words, said plastics are integrally moulded and devoid of discontinuity points without neither welding borders nor mating surfaces. In this manner, objects like RFID tags or wireless electronic devices presenting the injected external housing of the invention have enhanced properties during operation in high temperature and/or harsh chemical environments.
Furthermore, according to an embodiment of the invention, in case a through hole is needed in said objects and thus said assembly is provided with a further through hole, the injection nozzle is positioned in a protrusion of a half-mould facing a further protrusion of the other half-mould defining an injection chamber therebetween. Positioning said protrusions inside said further through hole, plastics from said chamber flow into said cavity and encapsulate portions of said assembly around said further through hole, manufacturing at the same time said through hole. As a result, an external housing is obtained as a single continuous body provided with a through hole in only one single shot process step. Consequently, said plastics are integrally moulded and devoid of discontinuity points at said through hole without being necessary a previous or subsequent manufacturing process step to obtain said through hole.